The invention relates to a process for the production of polyvinyl chloride having improved processability, especially in injection molding.
The past is replete with efforts to improve the processability of vinyl chloride polymers, i.e., to lower the viscosity of the polymer melt which, at a given temperature and shear rate, is dependent on the average molecular weight and the molecular weight distribution of the polyvinyl chloride. Thus, by using polyvinyl chloride of a low average molecular weight with a narrow molecular weight distribution, readily flowing polymer melts can be obtained but, as is known, these exhibit poor mechanical properties as finished components [Pol. Eng. Sci 17: 194 et seq. (1977)]. Processes have also become known for broadening the molecular weight distribution of polyvinyl chloride. Thus, according to DOS No. 2,651,532, a mechanical mixture of polyvinyl chloride powders with differing average molecular weights is prepared. This process, however, is expensive when conducted on an industrial basis. Furthermore, it yields powders which can be processed by injection-molding machines into smooth finished-part surfaces only if the average molecular weights of the mixed ingredients are not too different. The attainable reduction in melt viscosity is, however, small.
In contrast thereto, by mixing two powders having greatly differing average molecular weights, polyvinyl chloride finished parts are obtained with rough surfaces and flow lines. The cause is the heterogeneity of the powder. The very low molecular weight polyvinyl chloride grains melt more rapidly in the plasticizing zone of the injection-molding machine than the high-molecular weight polyvinyl chloride grains. This reduces the melt viscosity and similarly to the results of an excessive addition of a mold release agent, thereby impairs the plasticizing process for the high-molecular weight polyvinyl chloride grains, leading to the flaws in the finished part.
The disadvantage of using powder heterogeneity to affect molecular weight is avoided in the process of U.S. Pat. No. 4,072,806. This method varies molecular weight in vinyl chloride suspension polymerization by the simultaneous presence of regulating materials (lowering molecular weight) and branching agents (increasing the molecular weight). The thus-attainable reduction in polyvinyl chloride melt viscosity is linked to the amounts of regulating and branching components used as starting materials. The higher the quantities, the lower the melt viscosity. However, as has been borne out by experiments, using powdered material having reduced melt viscosity in accordance with this process, it is impossible to produce by the injection molding technique molded polyvinyl chloride articles having a flawlessly smooth surface characteristic. The cause of this resides in the nature of the branching component. The latter also leads to crosslinking of the macromolecules during the vinyl chloride polymerization when used in the high quantitites necessary for reducing melt viscosity. This can be readily investigated by preparing a solution of such a polyvinyl chloride powder in, for example, cyclohexanone because the crosslinked macromolecules are left therein as insoluble gel particles. Such crosslinked proportions in the powder prevent the production of the desired, smooth, flow-line-free finished surface during injection molding.
This drawback has been avoided in the process according to U.S. Pat. No. 3,956,985 which refrains from using branching components. A broadening of the molecular weight distribution is obtained therein by polymerization at at least two different temperatures, optionally in the presence of a compound having a regulating effect on the molecular weight. These regulators are added at the beginning of polymerization in this process.
The thus-attained reduction in melt viscosity for products having the same average molecular weight, is however, merely about 10%. Additional improvement is needed.